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THE   BEAVER                                   467

The caecum is disposed in a spiral.     The  teats  are   three  pairs*
non-inguinal in position.
The size of these squirrels is 16 to 18 inches exclusive of
the tail, which is longer. These animals can make an exceedingly
long jump with the help of their flying membrane. Nearly eighty
yards is the longest distance given for these aerial excursions. It
is stated that they are able to steer themselves to some extent
while in the air. As appears to be the case with so many
Rodents,'these animals feed largely upon beetles and other insects,
besides bark, nuts, etc.
The allied genus Sciitropterus has a much wider range. It
extends into the Palaearctic region and into North. America,
besides being found in India. There is here no membrane reach-
ing to the tail.- The palms and soles are furry. The caecum is
very much shorter, and so is the large intestine. The latter, in
S. volzicella, is not more than one-third of the length of the small
intestine. In other features there are no remarkable differences
in structure, save that the mammae, always three pairs, may be
inguinal.
Of the genus J&ugetaurus1- but a single species is known, which
is limited to high altitudes at Gilgit and perhaps in Thibet. Its
principal difference from the other genera of Flying Squirrels is that
the molars are hypselodont instead of brachyodont. The inter-
femoral membrane is rudimentary or wanting. The one species
is JB. cinereus. It is thought to live " on rocks, perhaps among
precipices." Dr. Tullberg attributes the hypselodont teeth to the
fact that the mosses *rpon which it is believed to feed may have
much sand and earth intermingled, which would naturally lead to
a more rapid wearing away of the teeth, and hence a need for a
good supply of dental tissue to meet this destruction.
Vain. 3. Castoridae.—This, the third family of the Sciuro-
morpha, contains but one genus, Oastor, the Beaver, with at most
two species, onen3STorth American, the other European. This large
Rodent has small eyes and ears, as befits an aquatic animal, and the
tail is exceedingly broad and covered with scales; the transverse
processes of the caudal vertebrae, in order better to support the
thick tissues lying outside them, are divided in the middle of
the series into two. The hind-feet are much larger than the fore-
feet, and are more webbed than in any other aquatic Rodent*
1 Thomas, J. Asiat. Soc. Bengal, Ivii, 1888, ]> 256,